Transmission pyrometry is a common mode of detecting the thermal state of a substrate. Thermal processing chambers commonly expose a substrate to intense, non-coherent or coherent radiation to raise the temperature of the substrate, either of the whole substrate or a part or surface area of the substrate. The radiation used to heat the substrate creates a strong background radiation environment in the chamber.
High power radiation is used to detect the thermal state of the substrate because it can be differentiated from the background radiation in the chamber. Lasers are typically used because they offer high power, and because they afford the opportunity to select a particular wavelength best suited to the substrate. Lasers produce coherent radiation that, when transmitted through a substrate, can indicate a thermal state of the substrate, which may be registered as a temperature. The transmitted radiation may be detected by a pyrometer and compared to the incident radiation, and the transmission is correlated to the substrate thermal state.
Radiation generated by lasers typically has a very narrow spectral width, and the precise wavelength of the radiation varies detectably as the laser operates. Temperature of the lasing medium affects the wavelength emitted by the laser, but even temperature-controlled lasers exhibit noise due to, for example, mode-hopping. As this varying radiation impacts a substrate, some of the radiation reflects between the opposite surfaces of the substrate, producing an interference effect. As the wavelength of the laser light varies, the combined effect of the laser light and the interference produces a great deal of noise in the transmitted light that reduces the ability of the pyrometer to detect the thermal state of the substrate with accuracy.
Combined with the noise inherent in the laser radiation is the effect of temperature on the substrate. As the substrate temperature changes, its refractive index may change, and its thickness may change, altering the interference patterns observed. These combined noise sources greatly reduce the ability to correlate transmitted radiation to thermal state, because the intensity of the transmitted radiation is modulated by varying interference effects.
Thus, there is a need for apparatus and methods of low-noise transmission measurement.